During the combustion process, gasoline is oxidized and hydrogen (H) and carbon (C) combine with air. Various chemical compounds are formed including carbon dioxide (CO2), water (H2O), carbon monoxide (CO), nitrogen oxides (NOx), unburned hydrocarbons (HC), sulfur oxides (SOx), and other compounds.
Automobile exhaust systems include a catalytic converter that reduces exhaust emissions by chemically converting the exhaust gas into carbon dioxide (CO2), nitrogen (N), and water (H2O). Exhaust gas oxygen sensors generate signals indicating the oxygen content of the exhaust gas. An inlet or pre-catalyst oxygen sensor monitors the oxygen level associated with an inlet exhaust stream of the catalytic converter. This inlet O2 sensor is also the primary feedback mechanism that maintains the air-to-fuel (A/F) ratio of the engine at the chemically correct or stoichiometric A/F ratio that is needed to support the catalytic conversion processes. An outlet or post-catalyst oxygen sensor monitors the oxygen level associated with an outlet exhaust stream of the catalytic converter. The post-O2 sensor signal is used for secondary A/F ratio control.
System diagnostics monitor the function of the oxygen sensors and the catalytic converter to ensure proper operation. Traditionally, diagnostics employ separate intrusive tests that evaluate the operation of the sensors and the catalytic converter. During the intrusive tests, the A/F ratio is manipulated and the sensor response is monitored. However, multiple intrusive tests may increase exhaust emissions and/or cause engine instability and reduced driveability that may be noticeable by a vehicle operator. Further, traditional diagnostics are more complex and computationally intense than desired.